Compressor-expander



March 30, 1965 LAMPKE 3,176,224

COMPRES SOB-EXPANDER Filed Nov. 25. 1960 3 Sheets-Sheet l sAMPLE 35 INTEGRATOR and PAM sTORAGE IcoMgI ssED DIFFERENTIAL PHASE SAW TOOTH TIMING OOMPARATOR SPLITTER GENERATOR Tfi lkF 27 25 so I 3/ I II V w WAVE TRIANGUL-AR f GENERATOR WAVE x32 33 GENERATOR F27 TIMING sAMPLE PAM INTEGRATOR and STORAGE (compressed) 25 29 28 f s 5 5 27 PAM D|FFERENT|AL 8' PHASE SAW TOOTH TIMING S|GNAL'-AT COMPARATOR 'NTEGRATOR SPLITTER GENERATOR 1. PRF

4/ 12 30 -31 sOuARE PuLsE "W"WA E SEQ' GENERATOR GENERATOR GENERATOR P27 132 TIMING 38 37 S REsET F/G. 3 34 OuTPuT IL e g 35 (n rmcIl pom A sIgnul) (compressed) PAM sIGNAL GATE wAvE FORM INVENTOR GEORGE 6. LA MPK E A TTORNE Y March 30, 1965 G G. LAMPKE 3,176,224

COMPRESSOR-EXPANDER Filed Nov. 25. 1960 3 Sheets-Sheet 2 RESET FIG. 4

(normal) PAM SIGNAL SAW TOOTH WAV EFOR M reset voltage 0 1 2h nt INVENTOR GEORGE 6. LAMP/(E By WW A TTOR/VEY March 30, 1965 a. e. LAMPKE COMPRESSOR-EXPANDER 3 Sheets-Sheet 3 Filed Nov. 25. 1960 reset voltage PC M. MOD.

TRANS- RECEIVER 1- MITTER T l M E Expander /6 Q it PAM 0 Compress PA M :MULTIPLEX DEMOD MULTI- CHANNEL AUDIO SOURCES MULTI- CHANNEL DETECTORS l/VVENTUR GEORGE G LAMPKE ATTORNEY 3,176,224 Patented Mar. 30, 1965 3,176,224 CGMPRESSOR EXPANDER George G. Lampke, Framingham, Mass, assignor to Raytheon Company, Lexington, Mass, a corporation of Delaware Filed Nov. 25, 1960, Ser. No. 71,609

Claims. (Cl. 325-38) This invention relates to pulse signal transmission systems, and particularly to volume range compression and expansion arrangements for reducingthe effects of noise 1 in such systems.

Pulse code modulation systems utilizing pulse amplitude modulation (PAM) of the signal energy employing a compressor and expander (compandor) arrangement in order to provide a better signal-to-noise ratio are known. Compandor arrangements have long been utilized in connection with ordinary signal transmission, and more recently in PCM systems wherein the modulator unit opcrates into a circuit having a compression characteristic for the compression of the volume range of audio or speech energy signal variation at the transmitter so as not to exceed the total transmitter energy range, and the demodulator system operates into a circuit having an expansion characteristic complementary to the compression characteristic for providing an overall linearity in the PCM system.

In the prior art ordinary signal transmission systems,

it has proved advantageous and desirable to transmit faithfully signals having an extremely wide range of volumes, such as music, by means of a transmissionmedium which itself possesses a limited volume range. The range of signal volumes which can be satisfactorily transmitted is limited by the characteristics of the transmission medium used and those of the transmission apparatus which may i be employed therewith. Such limitation by the transmission medium or apparatus in connection with ordinary signal transmission, is equally or even more of a limiting condition in PCM systems. In order to prevent signal distortion, it is necessary that the minimum transmitted signal amplitudes be maintained above that of the noise or other interference introduced by the transmission medium or apparatus, and that the maximum transmitted signal volumes be maintained below that value which would cause transmission apparatus, such as repeaters, in

the system, to overload. 1

Various compandors have been proposed for accomplishin-g this undistorted transmission for ordinary signals,

and certain of such devices have been extended or employed in PCM systems with varying degrees of effective ness. In general theory, corrrpandor arrangements function at the transmitting end of the system to relatively the invention.

over-amplify the smaller amplitudes of the signals until they are large with. respect to the undesired noise amplitudes and to undereamplify the larger amplitudes'of the signals so that they do not overload .the transmission medium or the apparatu therein, thus effectively compressing the volume range of the signals to bring them within the volume transmission range of the transmission medium before transmitting them thereover; By employing similar apparatus operating in a reverse complementary manner at the receiving or demodulating end of the system, the volume range of the received signals i effectively expanded to restore their original amplitude relations. A compandor arrangement thus provides a better signal-to-noise ratio to the overall-system. As

applied to PCM systems utilizing PAM transmission the expansion of a low volume range or amplitude level trans-- mitted signal is particularly advantageous in that when such signal is later compressed a better overall signal-tonoise ratio is obtained since the'noise level is also compressed.-

element of the compressor of FIG. 2;

Prior art compandors for PAM signals, following the principle developed with the earlier ordinary signal transof diodes either of the vacuum type, or more particularly 'of the semiconductor type, may result in undesirable overall signaldegradation due to non-uniformity of the temperature stability properties and/or the characteristic 10- curves of the diodes usedat each end of the transmission circuit. 1

Accordingly, the principal object of the present invention is to reduce the signal distortion in PAM systems employing a compandor arrangement by eliminating the use of diodes.

A further object of the present invention is to afford better temperature stability and the elimination of the matching requirements of diode characteristic curves in complementary volume range compressing and expanding circuits.

The above objects are accomplished in accordance with the present invention by utilizing in place of the diode circuits of'the prior art, ramp or slope functions which are generated or derived from, and in timed sequence with, the cyclic pulse repetition rate of the PCM system. The compression and expansion transfer characteristics thus provided willbe parabolic rather than the ideal logarithmic configuration. However, the improved temperature stability and slope uniformity produces an overall improvement in the system.

The above-mentioned objects, and various additional features and advantages of the invention will become apparent from the following detailed description of illustrative embodiments thereof taken, in conjunction with the accompanying drawings, in which:

FIG. 1 is a block diagram of the expander arrangement according to the invention;

FIG. 2 is a block diagram of the complementary compressor arrangement;

FIG. 3 is a basic circuit diagram of a differential comparator which'may be utilized within the corresponding expander block element of FIG. 1;

FIG. is a basic circuit diagram of a differential comparator which may be usedwithin the corresponding block FIG. 5 is a series of related Waveforms useful in understanding the operation of the expanding and compressing I steps, circuits, and arrangements of the invention; and

FIG. 6 is a simplified block diagram of a PCM system incorporating the compressor-expander arrangements of Referring firstto FIGURE 6, a typicalPCM multichannel ,system is illustrated in block form which employs a compandor arrangement consisting of the signal presdistorting compressor unit 10 located in the PCM transmitter 11, and the expander unit 13 which restores the signal linearity located in the PCM receiver 14. The

general operation of the system providesthat the transmitter llmay have a plurality of separate audio sources 15 fed to the audio modulator 16 wherein the 1 changes of audio amplitude for each channel are translated into changes of pulse amplitudes. These pulses are then interleaved for application to the compressor 10. For example there may be 24 separate sources of audio signals applied to the modulator 16 to produce a 24 channel 1 PAM signal which is applied to compressor 10 for compression of signal variations as required in the particular transmission medium accordance with the character isticcurve 17. The compressed PAM pulse train is then applied to theflPCM modulator 18. for translation to a predetermined PCM pulse train and application therelevels a,;; ,b c, d, I and f e'i of. FIG, 5+1. 7 to comparator.24 overthe lead 26,. is-ap siti e-going from to the transmitter 11 for application over the transinission link '19. Transmissionlink 19 may bee-radio, 3

frequency space link, a .video frequency cable link, or the like. r

Energy is received from the transmission mediu ni 19 f and appliedstherefrom to the'PCM deniodulatofltl by the receiver Demodulator. 20 operates to translate 'the PCM pulse train into a iPAM pulse train'. a This PAM signal is then applied to the expander 13to remove the'distortion introduced at compressor 19 byacircuit i and 21 are maintained uniform and'stable as reversed c'omplements ofoneanother, the overall linearity of the 1' system will have been maintained and the channelipulses f of the PAM signal will then. be separated-into their 'respective channels bymultiplex' demodulator 122," and each will' be applied-,to an vappropriate audio detector within 7 the multi-channel detecting means 23 for faithful reproduction or the audio information fed into the systemby the sources15; i

' going sawtooth. Wave; and

over the leads 30 and 31, respectively, as inputs to the tri- I both .of'such wave s are fed angular wave generator shown as block element 32.. Generator 32 basically combines the positive-going and negative-going sawtoothwaves within a gate, which gate bears the proper time relationship to thesystem pulse repetition frequency.. Both linearsawtooth inputs to the gen- 'erator '32 will be matched; since they are derived by passing through phaseysplitter 2.9. suitable.- type of having afiOmplementary characteristic with respect to a the compres'sion chairacteristicgfl, assubstantially shown bythe characteristic curve 21; Provided that curves 17 linear sawtooth waveforrn' generator maybe utilized within biock element 28. fm example a Miller integrator a Bootstrap integrator, a current-driven capacitor, or a sim-,

ple R+Cnetwork with appropriate selectionof the linear It will be'app arent to thoseslcille'd in the art, however,

that any variation. in the characteristic curves 1701" 21 will introduce distortion Within the system Whensuch curves areproduced'by-diode characteristics, as linthe prior art, variations in the slope ofone' orboth curves may occur due to the non-uniformity of the diode e16, v meme utilized to an extent that is undesirable for faithful' V 'reproduction of the audio signals transmitted over the sYStem. r s

portions" of :thefderived transient voltage. 7

I Obviously, the triangular waveformdeveloped in generatorifil may be'o'f either positive or negative polarity 'depending' upon the D.C. level established and the type of gate employed therein. Anyfknown transistor or diode gate may-he utilized within generator32. The output from'triangular generator 32 is fed as-aninput to the W wave generator33 which produces the third input Wave C for comparator 24. The W-shaped fwaveproduced within'gener'ator 33 may also be' generated 'in a -=number of ways. The time relationship of input wave C,

as shown inFIG. 5-3, is established 'by means of the timing inputover lead 2 7.g One suitable method of gen- To overcome undesirable variations in the character- 'istic curves 17 and 21, the. present invention derives such characteristic curves from swee'pcircuits and integrators, and thus fsub'stit'utes a modified sawtooth slope or ramp' jfuncti'o n fort-hat of theusu'al ,di'ode characteristic. The I'basic' operations of this, type of expander 13and compressorlfl are showni'in' the blockfdiagramsof FIGS. 1

and 2 ,;respectively Bearing 'in mind the fact that the.

iLPAM' signals fed into expander 13 are distortedsuch 7 that the pulsesproduced by the nominal: audio levels at the sources 15; have'ibeen. altered for passage through transmission'link 19, and .thus pu lses representing highest soundgvolumes have been reduced. in. amplitude while pulsesrepresenting'lowest'sound volumes have been increased in amplitude by operation of compressor 10 in ccordance.with thecharacteristiccurve 17 .thereof,' the .necessary expansion withingelement13101 restoring the 7 -signal'levels willfirst'be described. .1 2 Asshown'in FIG; 1; the'actua1'expansion is'pe'rformed by the differential comparator 24 and-the integrator 25;

pared, and a thirdinput "C 'w hich acts as a current drive. a

erationof this W-shapedv'vave consists of the concurrent application of the triangular waveform developed ingen- .erator 32'anda square wavevpulse of proper polarity and timingas inputs to anfOR. gate of a simple diode or -most positive or' negative; input level. I.

" transistor type; The output ofsuch gate may assume the vlntegrator 25 is actuallyincorporated within the comparator 24as shown in'detailin a preferred circuit em [bodiment at FIG. 3; such inte gration function is produced bythe capacitor 34 which-is connected in parallel with theoutputterminal 35'fro m' which the expanded PAM signal is fed'to the PCM'multiplex demodulator element 2 2 of FIG. 6. The driving current for comparator 24is supplied as shown by the waveform C connected to the base" element. of'transistor' circuit member TR -S of FIG. 3'. This current flowsprincipally fromthe base of TRES through a resistor 36which is connected between 'the emitter element of TR-3 and a voltagesource of the leveljH-E, the value of which voltage is indicated in FIG.

.50 The'expandercomparator zd shown indetail in FIG, 3, has three inputs'jjtwo inputs 'A and B'which are com- 7 5 -3 current fiowing'through the'resistor impresses I a voltageacrosssuch resistor which in turn provides a driving current whichmay flow; via the emitter-collector path of transistor TR'3', through one sorjthe other of tran- {sistor circuit members :TR-I and'TR-Z. f Such driving current i, which is indicatedin FIG. 3, fiows through the transistor circuit member .TR-Z. onlyrwhen the base ele- -m'e1ito fTR-2 is more negative inrvalue than the base of i the transistorrcircuit member TR.1. llAs indicated iii-FIG.

The waveforms ofgsuch threerinputs are illustrated in' FIG j as 5- 1, 5-2, and 5fl3,'respiactively. Input, A. Qftothe expander ofFIG. 1 is'Ithe compressed PAM sig- 1 naldevelopedin, the demodulator 20 of FIG.:6,' and will have amplitude variations inthe range between the limits' bfQ-l-Efafnd. -E, as shown by the arbitrary amplitude p'u faPp1 j linear sawtooth wave as shown in FIG. 5- -2jwhich varies fbetween'the limitsof s-EIand +E. duringjthe pulse tim- I throughTR-l.

ingperiod commencing at 0- andcontinuing through 2t as indicated 'insuch figure TThe 'posit'ive sawtooth wave input Biscyclically derived by the. sawtooth generator I 28at a ratejand timing period which'is determined by ,theinverseiof the pulse. repetition frequency of the PCM" system. lead 27 indicates the appropriate synchronizing timing inputto generator 33 which-may betaken from eitheer'eceiver 14 'ordemodulator 2t v nite phase splitter 29,.coupled s the-.oupu

t new *3,; the bas'efof TR -lyis connected toith esource of comcated' as waveform AKFIG; 5 l'),.while the remaining pressed PAM "signals 7 which 1vary-within the range indi f inputt'o thercomparator- 24,. the positive going sawtooth wave B is ,connect'ed'to the base element dffTR-Z. The

" emitter elements of TR-l and TR 2 are interconnected to each otherand tothe collectorelementofTk-li. There'- ffore the*current i; supplied' by the vVii-shaped wave C,

. ;flo'wsfthroug-h TR-fz 'whenitsl base element is inorenegadicated' in FIG. 5. "However;

' accordance with the current-flowing;

'to'othgeneratdr 28,. provides the aforesaidfwave:B 'over' 1 thelead Phase splitter 29' als'o .provi des a negative Assumethat em, a compressed.PAMi-signal applied to I the base element of TR-I of +13 volts;is-applied at time t.0; Thus over, thefentire time perio'd' fro'm" 0 tof2i curve 21 of expander 13 at FIG. 6.

5 shown in FIG. 5, i.e., t 2t the waveform B which is applied as the voltage e to the base element of TR-2 will always be more negative than waveform A. During thistime, therefore, TR2 will remain ON and TR-l OFF. The current flowing through TR-2 during such period may be written as the expression:

from to 2: e will change as follows:

21 ec f z'dt Equation 2) C34 0 i It can be proven that the output at terminal 3-5 will be asymmetrical about the time t if waveform Cis symmetrical, and it will have the form shown in FIG. -4.

Such output voltage is derived from the connection to the collector element of TR-2, and is designated above as the quantity e as indicated in FIG.'3. The collector element of TR-1 is returned to the negative voltage source, also as indicated in FIG. 3. It will now be recognized that the waveform or characteristic at the collector of 'I'R-2, e has the desired expansion characteristic shown in the Voltage e will follow a square law curve center at t This curvature may be altered by adjusting AE or K, Where K is the slope of the W-shaped wave C, and AB is the difference between such wave and the voltage +E', as shown in FIG. 5 -3. The slope at t of the -characteristic expansion curve, e

-shown in FIG. 54, may be made Zero if AE becomes 0 at the time t=t thus obtaining infinite expansion. As

the PAM signal amplitude varies'betweenthe' levels of +E and -E, the output will vary as shown by the typical developed paths a, b, c, d and e of FIGQSM. The output voltage 2 may be stretched by' sampling and'storing the signal amplitudes at the time t=2t until, a later time such as that indicated by the dashed line at t=n't in FIG. 54,' by means of the optional sampling and storage element 39 shown in FIG. 1, which block element would physically be inserted before the output terminal 35. as designated by the dotted line arrow shown at the location X in FIG. 3.

' The pro-distortion provided by the compressor of FIG. 6 in accordance with the -curve 17 is effected by -means of. the compressor circuit shown in block form in FIG.2. The block elements shown in FIG. 2 are similar in function, characteristic, and construction to those of the similar elements previously described in connection with FIG. 1. The basic preferred electrical embodiment of such comparator is shown in'FIG. 4, with the corresponding input waveforms illustrated in FIGS. 5'5, 56 and 57.

It willbenoted that FIG. 4 is identical electrically, with FIG. 3, and the compressor comparator circuit 24 is thus the sameas that of the expander described'above, except that different signals are appliedto the several transistor bases and a different output is'derived. The compression .is performed by the differential comparator 24and the integrator 25. The compressor comparator 24, shown in detail in FIG. 4,-has three inputs; two inputs A and B which are compared, and a third input Cfwhich acts as a current drive; The waveforms of suchthreeinputs are illustrated inFIG. 5 as 5 -5, 56, and 5- -7, respectively.

Input A to the compressor of FIG. 2 is the normal PAM signal developed in the modulator 16 of FIG. 6, r

and will have amplitude variations in the range between the limits of +13 and E, as shown by thearbitrary fed through integrated W-shaped wave is shown in FIG. 56, and it will be noted that the overall configuration thereof agrees with that of the wave shown in FIG. 54. The basic W-shaped wave produced within generator 33 may be derived in exactly the same manner as that described above with respect to the expander of FIG. 1, by means of the block elements 28, 29 and 32, and the related signal leads 27, 30 and 31. Integrator 25 which follows generator33 may consist of a simple capacitor circuit element as in the case of the capacitor 34 of the integrators 25, or may optionally comprise a more sophisticated combination of circuit elements, as is known in the art.

The third input wave C for the compressor comparator 24 consists of a square pulse gate as shown in FIG. 5--7. Wave C is developed in generator 42 and consists of a square wave pulse of proper polarity, timing, and magnitude in relation to the system pulse repetition frequency. The amplitude of such gate or square wave pulse affects the overall gain of the compressed signal, and may be chosen at a predetermined level with respect to the most positive or negative input level. As indicated in FIG. 57, in the preferred embodiment the amplitude of the square pulse gate wave C of the compressor 10 is chosen to be of the same order as that of the W-shaped wave C which provides the current drive for the comparator circuit of the expander 13.

7 As in the expander circuit embodiment of FIG. 3, the preferred circuit embodiment of the compressor which is shown in detail in FIG. 4, actually incorporates the integ'rator 25 within the comparator 24. Again such integration function is produced by a capacitor 34 which is connected in parallel with an output terminal 35' from which the compressed PAM signal is fed to the PCM modulator element 18 of FIG. 6. Also, as in the expander, the current generated by transistor circuit member TR-3 during the time period from Zero to 2t 'shown in FIG. 5, i.e., (l t b flows through the transistor circuit member TR-2 only during such times when the base element thereof is more negative in value than the base of the transistor circuit member TR-l. If the amplitude of thenormal PAM signal wave A which is connected as an input to the base element of TR-l has the value of +E volts (FIG. 55), then, as in the expander, an indicated current i will fiow through the compressor transistor TR-2 during the entire time period, for the reason that the waveform B' which is applied as the voltage, e' to the base element of TR-Z will always be more negative than waveform A. During this time ftherefore, the output voltage e' at terminal 35 will be the integral of such driving current, and may be written, in a manner analogous to that in Equation 3 above, as the expression:

I c2 034E556 -S1 1ch output voltage will have the form shown in FIG.

58.; It will be recognized that such wave, being based .upon the waveform B, is related to the desired com- 2 2! L (E-C)dt Equation 4 -pression characteristic shown in the curve 17 of com- 'pressor 10 at FIG. 6.

Voltage e .will be asymmetrical about the time t and'as the PAM signal amplitude is reduced the output from terminal 35' will vary as shown by the typical developed paths a, b, c,- d, and e of FIG. 58; Due to the shape of waveform B, low amplitude pulse signals will be expanded and high signals will be compressed. As previously'noted, the amplitude of wave C affects only the overall gain of the compressed signal. The compression characteristics are determinedby waveform B, and as in the case of the expander, the compressed signal may bestretched by sampling and storing the signal amplitudes beyond the time t=2t by means of an optional sampling and storage element 39 which would physical-' 9 4. A communication system according to claim 3 wherein said parabolic function characteristic curves expressed by:

are derived from slope function and fixed voltage means. I

5. In a pulse code modulation signal communication system comprising a transmitter having a compression device for compressing input modulated signals according to a given compression characteristic curve and a receiver for receiving said signal transmission, said receiver including an expansion device for expanding received signals according to an expansion characteristic curve substantially the complement of said compression characteristic curve, the method of generating said characteristic curves in said transmitter and receiver which comprises the steps of generating, independent of said input modulated signals, a sawtooth wave in timed relation to the cyclic pulse repetition rate of said system, and deriving a modified slope function waveform from said sawtooth wave.

6. A communication system according to claim 5 wherein the step of deriving a modified slope function waveform includes a square pulse generator means for providing a parabolic function characteristic curve.

7. In a pulse code modulation signal communication system comprising a transmitter having a compression device for compressing modulated signals according to a given compression characteristic curve and a receiver for receiving said signal transmission, said receiver including an expansion device for expanding received signals according to an expansion characteristic curve substantially the complement of said compression characteristic curve, the method'of compressing said modulated signals which includes the steps of generating a sawtooth wave in said transmitter in inverse timed relation to the cyclic pulse repetition frequency, deriving a modified slope function Waveform from said sawtooth wave, and differentially comparing such slope function waveform with said modulated signals.

8. A communication system according to claim 7 wherein the step of deriving a modified slope function waveform includes integrator means for providing a parabolic function characteristic curve.

9. In a pulse code modulation signal communication system comprising a transmitter having a compression device for compressing modulated signals according to a given compression characteristic curve and a receiver for receiving said signal transmission, said receiver including an expansion device for expanding received signals according to an expansion characteristic curve substantially the complement of said compression characteristic curve, the method of expanding said received signals which includes the steps of generating a sawtooth wave in said receiver in inverse timed relation to the cyclic pulse repetition frequency, deriving a modified slope function waveform from said sawtooth wave, and differentially comparing such slope function waveform with said received signals.

10. A communication system according to claim 9 wherein the step of deriving a modified slope function waveform includes comparator means for providing a parabolic function characteristic curve.

References Cited by the Examiner UNITED STATES PATENTS 2,220,689 11/40 Shore 179-1555 2,795,650 6/57 Levine 17915.6 2,803,702 8/57 Ville et a1 32538.1 2,816,267 12/57 De Jager et al 325-38.1 2,874,222 2/59 De Jager 17915.5 2,897,275 7/59 Bowers 32538.1

OTHER REFERENCES A Sampling Comparator, Electronic Engineering, December 1958, pages 685-389.

DAVID G. REDINBAUGH, Primary Examiner.

L. MILLER ANDRUS, Examiner. 

1. IN A PULSE CODE MODULATION SIGNAL COMMUNICATION SYSTEM COMPRISING A TRANSMITTER HAVING A COMPRESSION DEVICE FOR COMPRESSION INPUT MODULATED SIGNALS ACCORDING TO A GIVEN COMPRESSION CHARACTERISTIC CURVE AND A RECEIVER FOR RECEIVING SAID SIGNAL TRANSMISSION, SAID RECEIVER INCLUDING AN EXPANSION DEVICE FOR EXPANDING RECEIVED SIGNALS ACCORDING TO AN EXPANSION CHARACTERISTIC CURVE SUBSTANTIALLY THE COMPLEMENT OF SAID COMPRESSION CHARACTERISTIC CURVE, MEANS INCLUDING SAID COMPRESSION DEVICE TO GENERATE SAID CHARACTERISTIC CURVES IN SAID TRANSMITTER AND 